ABSTRACT Antibody-antigen complex (IC) deposition within and outside blood vessels that is associated with neutrophil infiltration is pathogenic in many human immune mediated disorders. The objective of this competing renewal remains the delineation of receptors and intracellular signals that regulate IC-mediated neutrophil recruitment and associated organ damage. Fc?Rs have emerged as key mediators of inflammation in a range of IgG-mediated diseases from glomerulonephritis, arthritis and vasculitis to fatal anaphylaxis. Our studies in mice expressing the uniquely human activating Fc?R, Fc?RIIA selectively on neutrophils of mice lacking their own activating Fc?Rs have shown that Fc?RIIA is a key molecular link between IC deposition, neutrophil influx and tissue damage. As such, this receptor must be tightly controlled. In the last funding cycle, we identified two major pathways regulating Fc?RIIA function. First, surprisingly, the neutrophil CD18 integrin Mac-1, known to promote inflammation, inhibited Fc?RIIA mediated neutrophil capture by immobilized ICs under flow by cis interaction of its ligand binding domain with Fc?RIIA's ectodomain. This may have relevance for human disease as the non-synonymous Mac-1 small nucleotide polymorphism (SNP) R77H associated with lupus and systemic sclerosis risk impaired Mac-1's affinity for its known ligand, complement C3 and for Fc?RIIA. Second, we showed that Fc?RIIA signaling to Abl-1/Src kinase and subsequent F-actin polymerization was required for rapid neutrophil arrest on IgG and the strengthening of Fc?RIIA-IgG bonds under shear flow in vitro. Accordingly, in vivo, 2- photon intravital microscopy (IVM) of the kidney revealed that in glomerular capillaries, intravascular ICs directly captured neutrophils via their Fc?RIIA without evidence of prior neutrophil rolling. This recruitment was inhibited by a Abl/Src inhibitor suggesting that it is an active process requiring intracellular signaling. Two related questions arise from these findings that are the basis of the current proposal. First, how is the interaction of Fc?RIIA-Mac-1 ectodomains regulated, does R77H perturb this interaction and how does this lateral interaction inhibit Fc?RIIA affinity for ligand? Second, what Fc?RIIA mediated intracellular pathways are engaged to promote capture of circulating neutrophils by deposited IgG? These questions will be addressed using knock-out mice, IVM and models of disease combined with fluorescent lifetime imaging microscopy (FLIM) analysis, biochemical assays and in vitro assays that mimic neutrophil recruitment to IC-coated endothelial cells under flow. We anticipate that the information provided by the proposed studies will give insights into how Fc?RIIA mediated neutrophil recruitment is regulated and thus identify important therapeutic leads for the prevention of Fc?RIIA mediated neutrophil accumulation, potentially one of the earliest events in IgG mediated inflammatory disorders.